The invention relates to a blood pump without bearing, operating according to the rotary pump principle, for temporary or long-term blood conveyance.
For temporary short-term blood conveyance extracorporeal blood pumps are used which comprise a rotationally driven impeller. Said impeller is supported on bearings in the pump casing. Examples of such blood pumps are described in EP 0 451 376 B1 and DE 43 21 260 C1. The impeller is driven via a magnetic coupling by a rotating rotor located outside the pump casing. The bearings supporting the impeller pose a problem in connection with the blood pump since thrombosis may occur at the bearings. Further, there is the danger of abrasive particles of the bearings contaminating the blood. Seals designed to protect the bearings against penetration of blood have also turned out to be unsuitable for the medium-term to long-term use (days to years). Blood pumps with mechanical support of the impeller are not suited for the long-term use for the aforementioned reasons. Pump systems having magnetic bearings (U.S. Pat. No. 5,385,581 A, DE 196 13 388 A1) which contactlessly support the impeller in an electromagnetic bearing means require a considerable controlling effort and a voluminous configuration because of the complex supporting structure where additional energy must be supplied to a large extent due to the active impeller centering.
It is an object of the invention to provide a blood pump having a rotor rotating in a pump casing where the danger of blood contamination and thrombosis minimized.
This object is solved according to the invention with the features stated in claim 1.
The blood pump according to the invention is a blood pump without bearing which is not provided with any mechanical bearings. The impeller is freely movable within a limited clearance in the pump casing. The impeller is rotated by an external magnetic driving means thus being self-centering. At least a front side of the blades comprises supporting surfaces which hydrodynamically lift the impeller during rotation. The static force of attraction of the permanent magnets in the impeller and the driving means tends to press the impeller against the pump casing wall facing the driving means. However, the supporting surfaces in the impeller cause the impeller to be lifted from the bottom surface during rotation such that the impeller slides on a blood cushion thus being kept at a distance from the wall. The impeller without bearing is passively centered in the pump casing via permanent magnets in combination with hydrodynamically acting driving forces. The lateral centering of the impeller is also effected by the magnets cooperating with the driving means. In this way it is possible to create a blood pump without bearing and shaft where the impeller is suspended in the pump casing.
The blood pump without bearing according to the invention offers the advantage that due to the fact that no bearings and sliding seals are provided the risk of thrombosis of the blood and penetration of foreign bodies into the blood is reduced. Thus the blood pump according to the invention cannot only be used as an extracorporeal blood pump for short-term application but also as an implantable blood pump for long-term operation. The blood pump is operable with high efficiency due to the low centering-induced losses wherein the required capacity lies in the range of 6 W under physiologically relevant operating conditions such that the pump has a long service life even when configured as a battery-operated portable device.
The impeller may comprise a straight continuous passage extending from the inlet to a bottom wall of the pump casing. Thus the impeller is provided with vanes on both sides.
Preferably, the impeller blades are arranged such that they protrude to opposite sides from the circumferential wall of a disk-shaped or cone-shaped supporting body. The impeller does not form a disk which would, together with the bottom wall of the pump casing, define a narrow gap. This also reduces the risk of thrombosis. In all areas of the pump casing a blood flow is maintained without there being the danger of dead water areas.
As seen from the top the blades are of essentially triangular configuration and comprise the blade-side magnets. The triangular form of the blades allows the blade volume to increase with increasing radius such that the fluid passage area available between the blades can be kept constant on all radii. Thus the conicality of the pump casing, which would be required to ensure that on all circumferential circles approximately the same volume is available, is reduced or eliminated.
The blood pump according to the invention is a centrifugal pump where the outlet is arranged essentially tangentially to the outer edge of the pump casing. Since the maximum pressure prevails in the outlet a radial force is produced which tends to press the impeller away from the outlet. To counteract this decentering force a peripheral ring diffusor is provided on the pump casing according to a preferred aspect of the invention, the ring diffusor ending in a tangential outlet. Said ring diffusor is a helical duct which causes the pressure prevailing in the outlet to be distributed over the circumference of the pump casing thus having a centering effect on the impeller.